Microfluidic technology is increasingly being used in a number of devices and applications. As just one example, microfluidic-based technology enables the formation of so-called “lab-on-a-chip” devices that perform complex analysis on fluid-based materials. These devices are often formed using conventional semiconductor processes, enabling the formation of small-sized devices. In many microfluidic-based devices, there is a need to pump or transport a fluid from one location to another. Micromechanical pumps, for example, have been developed to provide the pumping force within a microfluidic environment.
Bubble-driven valve-less micropumps are one attractive driving mechanism for microfluidic-based devices. Compared to micromechanical pumps with moving valves, their fabrication is simpler and reliability is generally higher, due to the absence of wear and tear. Typically, bubbles are formed using thermal generation (i.e., boiling the liquid). Unfortunately, this bubble-actuation method is energy intensive due to the high heat loss present in the microscale environment.
Bubble elimination is also a problem. Typically, vapor bubbles are eliminated by condensation. While bubble generation is activated by heating and fast (e.g., in microseconds for typical microdevices including thermal inkjet), bubble condensation is passive by cooling and far slower (e.g., milliseconds and slower for typical microdevices including thermal inkjet). As a result, the maximum achievable operation frequency of a thermal microdevice is dominated by cooling.
As for other bubble generation methods, removal of insoluble gas bubbles from a sealed device is harder—if even possible at all. This difficulty has been discouraging bubble-driven pumps from using bubble generation methods other than boiling (e.g., direct injection or electrolysis). As a result, most bubble-driven pumps are made having an open configuration (i.e., more like dispensers), such that the bubbles are expelled with the liquid in which they are carried.
Currently, there is no bubble-driven pump suitable to circulate liquid in a closed-loop microfluidic device. Such a pumping mechanism is desirable since many microfluidic devices are closed systems and require circulation of fluids within the system. Such a pump is especially attractive for fuel cells, because it could potentially utilize the existing gas bubbles—the inherent byproducts of any fuel-cell electrochemistry—to pump fuel. One fuel cell that should benefit from such a device is the micro Direct Methanol Fuel Cell (μDMFC).
There thus is a need for a bubble-based pumping mechanism that can effectuate the directional pumping of a liquid through bubble generation. In addition, the mechanism of the invention described herein would permit the removal of bubbles without having to condense the vapor or completely lose the underlying liquid medium. As a result, the invented bubble-driven micropump can utilize a variety of other gas generation schemes, including, for example, heating, electrolysis, injection, chemical reaction and cavitation. In this regard, the concerns of specific applications (e.g., energy efficiency, thermal sensitivity, bio-compatibility, adjustable flow rate, and the like), can be addressed by the flexibility of different gas generation methods.